In the field of graphic art, a method for forming images exhibiting photographic characteristics such that the contrast is quite high (particularly a gamma value of 10 or more) is needed to ensure good reproduction of images of continuous gradation comprising dot images or good reproduction of line images.
For this method, a special developer called a "lith developer" has heretofore been used. This lith developer contains only hydroquinone as a developing agent, and is greatly decreased in the concentration of free sulfite ions (usually not more than 0.1 mol/liter) by using a sulfurous acid salt as a preservative in the form of a formaldehyde adduct so as not to deteriorate the infectious developing properties of the lith developer. Therefore, the lith developer has a serious disadvantage in that it is quite easily subjected to air oxidation; thus, it cannot be stored for more than 3 days.
To obtain good photographic characteristics such as high contrast, a method of using hydrazine derivatives is known, as described, for example, in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857, 4,243,739, etc. This method permits the formation of images exhibiting photographic characteristics such as super high contrast and high sensitivity, and furthermore, since a high concentration of a sulfurous acid salt can be added to the developer to be used, the stability of the developer against air oxidation is greatly increased as compared with the lith developer.
The above method, however, causes the undesired phenomenon, e.g., formation of black spots due to infections development although it provides high sensitivity and high contrast, and produces a serious problem in the photo-mechanical process. The black spots are formed in non-exposed areas between halftone dots (sometimes called black pepper). This phenomenon becomes particularly conspicuous when the sulfite ion content in a developer is decreased or the pH of a developer increases due to fatigue of the developer which is stored for a long period of time, particularly under high temperature/high humidity condition, and it causes a significant reduction in photographic quality.
Various attempts have hitherto been made in order to eliminate the problem of black spots, but such improvements are frequently accompanied by reduction in sensitivity and gamma value. Thus, no means has been known for solving the problem of black spots without hindering the effect using the above-described hydrazine compounds to increase sensitivity and contrast.
It has been found as described in Japanese Patent Application No. 68873/85 (corresponding to U.S. patent application Ser. No. 846,679 filed on Apr. 1, 1985) that decreasing the pH of a film surface to not more than 5.8 by adding an acidic substance is effective in preventing the formation of black spots. However, it has been found that this method has some disadvantages: for example, (1) a coating solution having a low pH value is unstable as a result of coagulation due to the mutual action of gelatin or an acid polymer, (2) the coating layer is inhibited in gelatin hardening, and (3) the sensitivity is reduced.